A device for displaying and recording analog information is described in copending U.S. patent application Ser. No. 747,167 entitled "Tapered Resistor Device" now abandoned. In that device the non-uniform temperature profile produced by a tapered electrically resistive element, when it is electrically energized, interacts with a threshold-type thermally sensitive medium, for example, a liquid crystal. The interaction results in the creation of a visible strip or dot along or in the threshold-type thermally sensitive medium, with the length or position of the strip or dot related in a straightforward fashion to the magnitude of the current flowing through the tapered electrically resistive element, whereby the device displays or records information.
The tapered electrically resistive element of the device described is comprised of a tapered resistive film which has a constant resistivity over the temperature range of interest. For a gradual taper, the local temperature rise of the resistive metal film above ambient, T, is given by ##EQU1## IN WHICH A is a constant, I is the current flowing in the tapered resistive film, P.sub.s is the sheet resistivity of the tapered resistive film, and W is the taper width at the point of interest along the resistive film. Suppose a given resistive film has a 3:1 taper (W.sub.maximum /W.sub.minimum = 3); then, if the midpoint temperature rise to T.sub.0, the temperature rise at the narrow end of the resistive film is 4T.sub.0 and that at the wide end of the resistive film is 4T.sub.0 /9. If T.sub.0 represents the threshold temperature rise needed to create the desired thermographic indication in the threshold-type thermally sensitive medium, then any regions of the resistive film heated above T.sub.0 results in unnecessary power dissipation.